But other than that, why not just tell them to set
x-select-enable-clipboard ?

Thanks for reminding me: is there any reason x-select-enable-clipboard
isn't t by default?

Because it will do an incredibly annoying thing unless you also
reconfigure certain other settings:

If you want emacs cut and paste to act like other recent X11 apps:
(setq mouse-drag-copy-region nil)
(setq x-select-enable-primary nil
(setq x-select-enable-clipboard to t)
(setq select-active-regions to t)
(global-set-key [mouse-2] 'mouse-yank-primary)
Then C-w/M-w/C-y act very similarly to C-x/C-c/C-v in other apps -
if then you also want the keybindings to be similar, turn on cua-mode.

[Aside: I think select-active-regions has bitrotted slightly on trunk as
it's occasionally messing up (apart from the known issue with mousewheel
scrolling), though I haven't managed a repeatable test case yet.]